(1) FIELD OF THE INVENTION
This invention relates to a circuit for regulating the output voltage of a charge pump circuit and more particularly to using current feedback to provide analog control of the frequency of clock signals driving the charge pump circuit.
(2) Description of the Prior Art
Charge pump circuits are frequently used in EEPROM, Electronically Erasable and Programmable Read Only Memory, Circuits. These circuits supply the high voltages needed for erasing and reprogramming the Read Only Memories. These high voltages must be regulated in order to provide the proper voltages for the required application.
Many of the regulating methods used require a voltage reference and a voltage comparator which have the undesirable attribute of consuming extra power.
U.S. Pat. No. 5,553,030 to Tedrow et al. describes a charge pump circuit using comparator circuit comprising a differential amplifier circuit.
U.S. Pat. No. 5,812,017 to Golla et al. describes a charge pump voltage multiplier circuit using a voltage comparator.
U.S. Pat. No. 5,483,486 to Javanifard et al. describes a charge pump circuit using a voltage reference, a divider circuit, and a voltage controlled oscillator in regulating the output voltage.
U.S. Pat. No. 5,394,365 to Tsukikawa, U.S. Pat. No. 5,671,179 to Javanifard, and U.S. Pat. No. 5,781,473 to Javanifard et al. describe charge pump circuits.
U.S. Pat. No. 5,726,944 to Pelley et al. describe a charge pump circuit using a voltage regulation circuit comprising a band-gap voltage source.
U.S. Pat. No. 6,177,828 to Kang et al. describes a charge pump circuit wherein the voltage regulation is achieved by halting and restarting the charge pumping operation.
Charge pump circuits are frequently used to supply the higher voltages required for electronically erasing and writing EEPROM, Electronically Erasable Programmable Read Only Memory, circuits. The output voltage of the charge pump circuit must be regulated to maintain the proper voltage over the required range of operating load conditions. It is important to keep the power consumed by these voltage regulating circuits as low as possible.
FIG. 1 shows an example of a conventional means for providing voltage regulation for a charge pump circuit. FIG. 1 shows a block diagram of a conventional voltage regulated charge pump circuit showing a current controlled oscillator 10 driven by a constant current generator 28. The constant current generator 28 is connected to a primary voltage supply, VDD, and supplies a current to the input 26 of the current controlled oscillator 10. The current controlled oscillator 10 produces clock signals, CKIN, at clock outputs 23 which are fed to the clock inputs 24 of the charge pump circuit 12. FIG. 1 shows two clock outputs and two clock inputs for two clock signals however the actual number of clock outputs, clock inputs, and clock signals may be more than two or less than two. The clock frequency of the clock signals CKIN determine the output voltage, VHV, appearing at the output 16 of the charge pump circuit 12.
The output 16 of the charge pump circuit 12 is fed to the input 20 of a detecting circuit 14 which serves as a voltage divider producing a voltage proportional to voltage at the output 16 of the charge pump circuit 12 at the output 22 of the detecting circuit 14. The output 22 of the detector circuit 14 is connected to the gate of an NMOS transistor 18 connected between the output 16 of the charge pump circuit 12 and ground potential. When the voltage at the output 16 of the charge pump circuit 12 becomes too high the voltage at the output 22 of the detecting circuit 14 turn on the NMOS transistor 18 and the current through the NMOS transistor 18 decreases the voltage at the output 16 of the charge pump circuit 12 until the voltage at the output 22 of the detecting circuit 14 drops and the current in the NMOS transistor 18 is reduced or turned off. This varying current in the NMOS transistor 18 provides voltage regulation for the charge pump circuit 12 but has the disadvantage of the power consumed by the NMOS transistor 18.
FIG. 2 shows another example of a conventional means for providing voltage regulation for a charge pump circuit 12. FIG. 2 shows a block diagram of a conventional voltage regulated charge pump circuit showing an oscillator 11 controlled by a differential amplifier 34. The oscillator 11 produces clock signals, CKIN, at the clock outputs 23 of the oscillator 11 which are fed to the clock inputs 24 of the charge pump circuit 12. FIG. 2 shows two clock outputs and two clock inputs for two clock signals, however the actual number of clock outputs, clock inputs, and clock signals may be more than two or less than two. The clock frequency of the clock signals CKIN determine the output voltage, VHV, appearing at the output 16 of the charge pump circuit 12.
The output 16 of the charge pump circuit 12 is fed to the input 20 of a detecting circuit 14 which serves as a voltage divider producing a voltage proportional to voltage at the output 16 of the charge pump circuit 12 at the output 22 of the detecting circuit 14. The output 21 of the detector circuit 14 is connected to a first input 22 of the differential amplifier 34. The output 31 of a reference voltage source 30 is connected to a second input 32 of the differential amplifier 34. The output 35 of the differential amplifier 34 is connected to the control input 36 of the oscillator 11. When the voltage at the output 21 of the detecting circuit 14, supplied to the first input 22 of the differential amplifier 34, is less than the voltage supplied by the reference voltage source 30 to the second input 32 of the differential amplifier 34, the signal at the output 35 of the differential amplifier 34, fed to the control input 36 of the oscillator, turns the oscillator 11 on. When the oscillator 11 is on, the oscillator 11 supplies clock signals, CKIN, to the input 24 of the charge pump circuit 12, and the voltage at the output 16 of the charge pump circuit 12 increases. When voltage at the output 21 of the detecting circuit 14, supplied to the first input 22 of the differential amplifier 34, is greater than the voltage supplied by the reference voltage source 30 to the second input 32 of the differential amplifier 34, the signal at the output 35 of the differential amplifier 34, fed to the control input 36 of the oscillator 11, turns the oscillator 11 off. When the oscillator 11 is turned off clock signals, CKIN, are no longer supplied to the inputs 24 of the charge pump circuit 12, and the voltage at the output 16 of the charge pump circuit 12 decreases. This turning the oscillator on and off provides voltage regulation for the charge pump circuit 12 but has the disadvantages of the need to supply a reference voltage source, the need for a differential amplifier, and of the fluctuation of the voltage at the output 16 of the charge pump circuit 12 caused by turning the oscillator on and off.
It is a primary objective of this invention to provide voltage regulation for a charge pump circuit using analog control of a current controlled oscillator without the need for a reference voltage supply.
It is another primary objective of this invention to provide voltage regulation for a charge pump circuit using analog control of a current controlled oscillator without the need for a reference voltage supply over the full range of the output voltage of the charge pump circuit.
It is another primary objective of this invention to provide voltage regulation for a charge pump circuit using analog control of a current controlled oscillator without the need for a reference voltage supply when the output voltage of the charge pump circuit exceeds a critical output voltage.
These objectives are achieved using a constant current generator and a second current generator controlled by the output voltage of the charge pump circuit to supply a current controlled oscillator. The current controlled oscillator supplies clock signals to the charge pump circuit. The current from the constant current generator is divided between the input to the current controlled oscillator and the second current generator. The frequency of the clock signals is a second frequency when the current to the current controlled oscillator is equal to the full current supplied by the constant current generator and decreases as the current to the input of the current controlled oscillator decreases to a first frequency as the current to the current controlled oscillator becomes zero.
When the output voltage of the charge pump circuit increases the current in the second current generator increases, the current flowing into the current controlled oscillator decreases, the frequency of the clock signals decreases, and the output voltage of the charge pump circuit decreases. When the output voltage of the charge pump circuit decreases the current in the second current generator decreases, the current flowing into the current controlled oscillator increases, the frequency of the clock signals increases, and the output voltage of the charge pump circuit increases. This provides smooth voltage regulation for the voltage at the output of the charge pump circuit without the need for differential amplifiers or a reference voltage supply.